1. Field of the Invention
The present invention relates to a filter automatic adjustment apparatus and a filter automatic adjustment method which adjust a characteristic frequency of a filter having an adjustment function to a target frequency and a mobile telephone system which includes the filter and the filter automatic adjustment apparatus.
2. Related Art
Recently, in respect of communication instruments (for example, a mobile telephone), development is focused on low-power consumption and miniaturization, and various wireless communication instrument parts tend to be integrated. The same tendency refers to the filter and the filter is frequently incorporated into the mobile telephone.
Generally, in production process of an integrated circuit, a dispersion of a resistor ranges from minus ten-odd percent to plus ten-odd percent, a dispersion of a capacitor ranges from minus ten-odd percent to plus ten-odd percent, and dispersion in characteristic frequency of a filter having the resistor and the capacitor ranges minus 20 percent to plus 20 percent. Correcting the dispersion is an important problem in incorporating the filter into the mobile telephone.
The term of characteristic frequency means a center frequency f0 for a band-pass filter (BPF) and a cutoff frequency (frequency at −3 dB point) for a high-pass filter (HPF) and a low-pass filter (LPF).
Patent Document 1 (Japanese Patent Laid-Open No. 2002-76842) discloses a filter apparatus which adjusts a main filter (filter to be adjusted) by comparing phase of an input signal of a reference filter with phase of an output signal of the reference filter and feeding back the phase comparison result to the main filter. The filter apparatus measures dispersion of the CR product in the reference filter based on the phases of the input signal and output signal of the reference filter. Since the reference filter is similar to the main filter in a configuration, the dispersion of the CR product in main filter is known by measuring the dispersion of the CR product in the reference filter and the characteristic frequency of the main filter can be adjusted. Since the filter apparatus disclosed in Patent Document 1 is always operated, the filter apparatus can immediately adjust the main filter even if the characteristic frequencies of the reference filter and the main filter are shifted, for example, due to fluctuation in power supply.
Patent Document 2 (Japanese Patent Laid-Open No. 2004-172911) discloses a filter apparatus which measures dispersion of the CR product of a reference filter by comparing phase of an input signal of the reference filter with phase of an output signal of the reference filter and stores the phase comparison result in a register. The filter apparatus adjusts the main filter based on the phase comparison result stored in the register. The filter apparatus is powered off after the characteristic frequency of the main filter is adjusted once. Therefore, electric power consumption is prevented in a period during which the characteristic frequency of the main filter is not adjusted.
The filter apparatus of Patent Document 1 has an advantage that the characteristic frequency of the main filter is difficult to shift even if the fluctuation in power supply or the fluctuation in temperature is generated because the filter apparatus is always powered on to apply a feedback loop. However, the electric power is wasted in the reference filter, a phase difference comparator, and a control voltage generator which are not necessary to original wireless communication. Therefore, there is a problem from the viewpoint of the low power consumption.
In the mobile telephone into which the main filter and the filter apparatus are incorporated, when the power consumption of the filter apparatus is increased, a talk time of the mobile telephone in one battery charge is shortened. When the battery capacity is increased in order to secure the long talk time, miniaturization of the mobile telephone cannot be realized. In such circumstances, it is necessary to provide the filter apparatus in which the power consumption is low as much as possible.
The filter apparatus of Patent Document 2 is superior to the filter apparatus disclosed of Patent Document 1 in respect to achieve the low power consumption by turning off the internal blocks necessary to the filter adjustment after the adjustment is performed once. However, since the filter apparatus of Patent Document 2 measures the dispersion of the CR product in the reference filter having the same configuration as the main filter only once, when the reference filter has a bad noise characteristic or small input dynamic range, that is, when C (Carrier)/N (Noise) characteristic of the reference filter is bad, sometimes the measurement result is different from a correct value. When the measurement result of the incorrect value is used, the filter apparatus fails in adjusting of the main filter. Accordingly, the filter apparatus of Patent Document 2 can not accurately adjust the filter.